Fuel cell systems are increasingly being used as a power source in a wide variety of applications. Fuel cell systems have been proposed for use in power consumers such as vehicles as a replacement for internal combustion engines, for example. Such a system is disclosed in commonly owned U.S. patent application Ser. No. 10/418,536, hereby incorporated herein by reference in its entirety. Fuel cells may also be used as stationary electric power plants in buildings and residences, as portable power in video cameras, computers, and the like. Typically, the fuel cells generate electricity used to charge batteries or to provide power for an electric motor.
Fuel cells are electrochemical devices which combine a fuel such as hydrogen and an oxidant such as oxygen to produce electricity. The oxygen is typically supplied by an air stream. The hydrogen and oxygen combine to result in the formation of water. Other fuels can be used such as natural gas, methanol, gasoline, and coal-derived synthetic fuels, for example.
The term “fuel cell” is typically used to refer to either a single cell or a plurality of cells depending upon the context in which it is used. The plurality of cells is typically bundled together and arranged to form a stack with the plurality of cells commonly arranged in electrical series. Since single fuel cells can be assembled into stacks of varying sizes, systems can be designed to produce a desired energy output level providing flexibility of design for different applications.
A common type of fuel cell is known as a proton exchange membrane (PEM) fuel cell. The PEM fuel cell includes a unitized electrode assembly (UEA) disposed between a pair of bipolar plates. The UEA comprises anode and cathode diffusion media (DM), anode and cathode electrodes, and an electrolyte membrane. The order of the UEA components is critical for fuel cell operation. Respectively, the order of the components is: the anode DM, the anode electrode, the electrolyte membrane, the cathode electrode, and the cathode DM. The cathode and anode electrodes typically include a finely divided catalyst, such as platinum, supported on carbon particles and mixed with an ionomer. The electrolyte membrane may be disposed against a subgasket that outlines a periphery of the fuel cell plate. The subgasket may be a stiff film having electrical insulating properties, as desired. The DM facilitates a delivery of gaseous reactants, typically the hydrogen and the oxygen, to an active region defined by an inner edge of the subgasket of the UEA for an electrochemical fuel cell reaction. The DM also aids in the management of water byproduct within the fuel cell. The subgaskets separate the fuel cell into an anode side and a cathode side, and electrically insulate the anode side from the cathode side. A sealing portion disposed on the subgasket militates against the gaseous reactants from escaping the fuel cell. The UEA components can be laminated together to form the UEA.
The UEA is disposed between the pair of bipolar plates, which act as current collectors for the fuel cell. Each of the bipolar plates has ports and feed regions, for the supply and exhaust of the gaseous reactants. The feed regions act to distribute or collect the gaseous reactants within the fuel cell. The supply feed region includes supply ports that distribute the gaseous reactants from a supply manifold to the active region of the fuel cell via a flow field formed by a plurality of channels in the bipolar plate. The opposing or outlet end of the bipolar plate has an exhaust feed region that includes exhaust ports where collected gaseous reactants leave the fuel cell and enter an exhaust manifold. The subgasket may be used to provide electrical insulation between the bipolar plates. In the area of the feed regions, the stiffness of the subgasket is an important factor in producing the proper interface between the plurality of flow channels and the subgasket. The subgasket must be sufficiently stiff to militate against the subgasket intruding into and restricting flow in the plurality of channels forming the feed regions.
The stack, which can contain more than one hundred plates, is compressed, and the elements held together by any conventional fastening means and anchored to clamping plates at the ends of the stack. In order to militate against the undesirable leakage of fluids, the sealing portion is compressed between the plate assemblies. The sealing portion is disposed along a peripheral edge of both sides of the subgasket and may be integrally formed with the subgasket. The UEA is secured by the use of stack compression and a land formed in the plate, which corresponds to the sealing portion of the UEA. Prior art subgaskets have incorporated designs having a constant thickness from the active region of a fuel cell, across and past the sealing portion. The prior art subgaskets, despite being functional, may result in a shortened life of a fuel cell in many ways. The prior art subgaskets may be relatively thick (a thick subgasket) when compared to a thickness of the membrane. A contrast of thickness between the thick subgasket and the membrane may lead to a localized area of high compression. The localized areas of high compression may lead to crushed DM, cracked anodes or cathodes, plate deformation, and shearing of the electrolyte membrane, any of which may lead to a shortened life of the fuel cell or a poor performance thereof. Alternately, the prior art subgaskets may be relatively thin (a thin subgasket) when compared to a thickness of the membrane. A low contrast of thickness between the thin subgasket and the membrane may lead to an excessive deflection of the subgasket by a flow of reactant gases.
Generally, the life of the membrane may be shortened as a result of one of a UEA over-compression and a UEA under-compression. Swelling of the membrane may cause one of the UEA over-compression and the UEA under-compression. Electrolyte membranes require certain humidity levels within a fuel cell for proper operation. In anticipation of fuel cell start up or shut down, the humidity levels may be varied as desired for optimal performance of the fuel cell. Membranes within the fuel cell may absorb water, causing membrane dimensions to vary with humidity. Conversely, the subgaskets maintain excellent dimensional stability with variances in humidity. Particularly, repeated expansion and contraction of the membrane at the contact edge of a thick subgasket and the electrolyte membrane in the fuel cell may lead to shortened fuel cell life.
Over-compression may be caused by a swelling of the membrane, as well as manufacturing processes used to form the UEA. The swelling of the membrane may affect a length, a width, and a thickness of the membrane. The UEA over-compression may be caused by the thickness of the membrane increasing as a result of the swelling. The thickness of the membrane increasing as a result of the swelling creates a compressive load variance across the UEA. The compressive load variance across the UEA creates a stress concentration at the inner subgasket edge. The stress concentration at the inner subgasket edge negatively affects a life of the membrane. Additionally, the thickness of the membrane increasing as a result of the swelling may increase the compressive load on the UEA in the subgasket area, causing a permanent deformation of the bipolar plate and adjacent DM.
Under-compression may occur in a tenting region adjacent the inner subgasket edge. The tenting region is an area of the UEA adjacent the subgasket edge where the compressive load on the membrane is significantly reduced or eliminated. The DM may act to bridge the step formed by an inner edge thickness of the subgasket. The DM may flexibly conform across the step formed by an inner edge thickness of the subgasket, resulting in a wedge shaped span located within the tenting region. Upon humidification of the membrane, the length and the thickness of the membrane may increase. The humidified membrane may swell into the tenting region. As a result of the UEA under-compression, the membrane may buckle. A buckling of the membrane may cause one of the anode electrode and the cathode electrode formed thereon to crack.
It would be desirable to produce a UEA for a fuel cell having a subgasket, wherein the effects of stress on the membrane caused by the humidity change thereof are minimized to prolong an operating life of the UEA.